According to one procedure, a component made of composite material is produced from a fibre preform subjected to a polymerization cycle.
A preform here means a volume of fibres which are laid in a set arrangement, notably obtained by superposing layers of fibres on top of one another on a mould surface.
When the surface of the mould is a developable surface, it is possible to use layup machines which seek to apply to the mould fibre plies the dimensions of which may be as large as those of the component. In such cases, each layer may consist of a single ply.
When the surface of the mould is not a developable surface, use may be made of a fibre laying machine that allows a plurality of fibres lying side by side to be applied simultaneously. Thus, a fibre laying machine allows a strip of fibres to be applied in each pass.
As an alternative, a laying machine may allow several slivers of fibres to be applied simultaneously. Slivers of fibres mean a group of fibres. A sliver is generally planar and of narrow width, of the order of one centimeter wide or less. As an idea of an order of magnitude, a sliver of fibres may be of the order of 12 mm, 6 mm or 3 mm wide.
In the remainder of the description, a fibre means a longilinear element which may consist of a single fibre or of several fibres in the manner of a sliver.
An application surface corresponds to the surface of the mould in the case of the first layer of fibres or to the latest layer applied in the case of the other layers.
FIG. 1 depicts a mould 10 on which a preform 12 is formed using a fibre laying machine 14.
This machine 14 comprises a magazine of fibres 16, a mobile laying head 18 supported by a robot (not depicted) such as an articulated arm for example, and means 20 for conveying the fibres 22 from the magazine 16 to the laying head 18.
The laying head 18 comprises a roller 24 able to pivot about an axis of rotation 26 moving over the application surface 28 and means 30 for heating the application surface ahead of the roller 24 in order to activate the resin of the fibres 22 laid by the roller to encourage the fibres to adhere to the application surface so that they maintain their positions.
In the remainder of the description, the longitudinal direction means a direction parallel to the axis of rotation of the roller. A longitudinal plane corresponds to a plane containing the axis of rotation. A transverse plane corresponds to a plane perpendicular to the axis of rotation. A radial direction is a direction perpendicular to the axis of rotation.
According to an embodiment illustrated in FIG. 2, the roller 24 comprises a cylindrical body 32 of one piece with two stub shafts 34 at each end pivot-mounted in bearings 36 of a support 38 secured to the laying head. The stub shafts 34/bearings 36 guidance defines the axis of rotation 26.
The support 38 applies a force to the roller 24 in the direction of the application surface 28 so that the roller 24 applies, on a lower generatrix 40, a force to the fibres 22 that are to be laid forcing them towards the application surface 28.
The roller can pivot freely about the axis of rotation 26. Its movement of rotation about the axis of rotation 26 results from the rolling of the roller 24 along the application surface 28.
According to one embodiment, the cylindrical body 32 is made of an elastomer coated with a teflon film. The cylindrical body and the film are intimately bonded and cannot move relative to one another.
The magazine of fibres 16 also referred to as a creel comprises a plurality of reels of fibres. The magazine 16 comprises means for applying constant tension to each of the fibres regardless of the speed or acceleration of the laying head 18.
If the tension for a given fibre 22 is insufficient, it causes axial compression of the fibre 22 as it is compacted by the roller 24, causing the fibre to wrinkle as illustrated in FIG. 3A, and these wrinkles will remain included in the preform in the form of small corrugations which will impair the mechanical properties of the component produced.
If the tension in a given fibre 22 is too high, it will cause bridging across a hollow in the component, as illustrated in FIG. 3B, and this bridging will remain included in the preform and lead to a defect that will have an impact on the mechanical properties of the component produced.
The fibres are laid at an ambient temperature of 20° C. or above. That being the case, each fibre is paid out by the simultaneous combination of two effects:
The first effect stems from the rolling of the roller over the fibre and should in theory make it possible to pay out a quantity of fibre that is equal to the distance covered by the roller if the fibre were in contact with the roller only along the lower generatrix 40 of the roller.
The second effect stems from the adhesion of the fibre to the roller over a certain angle of wrap A (visible in FIGS. 3A and 3B).
This last effect generally leads to defects for the following reasons.
In general, the path of the laying head is not necessarily straight and may describe a curve. When that happens, for a given rate of travel of the centre of the roller, the rate of travel Vext of a first end of the roller is greater than the rate of travel Vint of a second end of the roller in a curve. Now, bearing in mind the second effect caused by the adhesion of the fibres to the roller, the tangential speed of each fibre is constant at the lower generatrix of the roller and equal to the rotational speed of the roller θ multiplied by the radius of the roller R.
Therefore, if Vext is greater than θ.R, the fibre Fext applied at this point is too highly tensioned, and this tends to cause defects of the bridging type as illustrated in FIG. 3B. At the same time, if Vint is less than θ.R, the fibre Fint applied at this point is insufficiently tensioned, and this tends to cause defects of the wrinkling type as illustrated in FIG. 3A.
To limit the appearance of such defects, one first solution is to apply the fibres dry, not preimpregnated. In that case, the fibres can slip on the roller and become suitably tensioned. However, this solution entails the use of other polymerization techniques which need to allow the fibres to become impregnated with a resin. These polymerization techniques are generally more complicated to perform. In addition, insofar as the fibres are not preimpregnated, they can slide relative to one another within the preform and fail to be correctly positioned in the end analysis.
A second solution might be to envisage reducing the angle of wrap A in order to reduce the friction forces between the fibres and the roller. However, this solution is difficult to put into practice because it entails changing the design of the laying head. In addition, a certain angle of wrap is needed in order to guide the fibres. Thus, reducing the angle of wrap leads to impaired precision with which the fibres can be positioned in the longitudinal direction.
Document U.S. Pat. No. 6,390,169 proposes a third solution which is to use a segmented roller 42 as illustrated in FIGS. 4A and 4B. Each segment of the roller 44 is dedicated to one fibre and can have a rotation speed that differs from those of the other segments. Thus, the rotation speed of each segment of the roller is adapted to suit the displacement speed of the point of the said segment in contact with the application surface. As a result, each fibre is correctly tensioned.
Even though it limits the risks of the appearance of defects of the bridging or wrinkling type, this solution is not entirely satisfactory because the roller is relatively complex. Moreover, because each segment has a large radius of the order of 35 mm, the roller is unable to apply the fibres correctly against the application surface if the latter has an indentation of a radius of curvature smaller than 35 mm.